The present invention relates to a pattern formation method performed in semiconductor fabrication or the like, and more particularly, it relates to a pattern formation method employing double patterning.
In accordance with the increased degree of integration and downsizing of semiconductor devices, there are increasing demands for further rapid development of lithography technique. Currently, pattern formation is carried out through photolithography using a light source of a mercury lamp, KrF excimer laser, ArF excimer laser or the like. Furthermore, use of F2 laser of a shorter wavelength has been examined, but since there are a large number of problems in exposure systems and resist materials, the development in the use of the F2 laser is now being suspended.
In these circumstances, a method designated as double patterning has been recently proposed for realizing further refinement with a conventional exposure wavelength. In this method, exposure is performed with a desired mask pattern divided into two photomasks, and thus, the pattern contrast is improved.
The resolution attained in lithography is defined as k1·λ/NA (wherein k1 is a process constant, λ is an exposure wavelength and NA is a numerical aperture of an exposure system). When the double patterning is employed, the pattern contrast is improved so as to largely reduce the value of the process constant k1, and hence, the resolution can be largely improved even with the same exposure wavelength.
Now, a conventional pattern formation method employing the double patterning will be described with reference to FIGS. 11A through 11D, 12A through 12D, 13A and 13B.
First, as shown in FIG. 11A, a hard mask 202 (made of, for example, a silicon nitride film) with a thickness of approximately 0.12 μm is formed on a semiconductor substrate 201.
Next, as shown in FIG. 11B, a first ArF resist film 203 with a thickness of approximately 0.15 μm is formed on the hard mask 202. Thereafter, first exposure is performed through a first photomask 204 with ArF excimer laser 205 with NA of 0.85. After the exposure, the first ArF resist film 203 is baked with a hot plate at a temperature of approximately 105° C. for 60 seconds.
Then, as shown in FIG. 11C, the first ArF resist film 203 is developed with a 2.38 wt % tetramethylammonium hydroxide developer, so as to form a first resist pattern 203a. 
Next, as shown in FIG. 11D, the hard mask 202 is etched with a fluorine-based gas or the like by using the first resist pattern 203a as a mask, so as to form a first hard mask pattern 202a. 
Subsequently, after removing the first resist pattern 203a by ashing with oxygen plasma as shown in FIG. 12A, a second ArF resist film 206 with a thickness of approximately 0.15 μm is formed on the first hard mask pattern 202a as shown in FIG. 12B.
Next, as shown in FIG. 12C, second exposure is performed through a second photomask 207 with the ArF excimer laser 205 with NA of 0.85. After the exposure, the first ArF resist film 203 is baked with a hot plate at a temperature of approximately 105° C. for 60 seconds.
Then, as shown in FIG. 12D, the second ArF resist film 206 is developed with a 2.38 wt % tetramethylammonium hydroxide developer, so as to form a second resist pattern 206a. 
Next, as shown in FIG. 13A, the first hard mask pattern 202a is etched with a fluorine-based gas or the like by using the second resist pattern 206a as a mask. Thereafter, as shown in FIG. 13B, the second resist pattern 206a is removed by the ashing with oxygen plasma, so as to obtain a second hard mask pattern 202b. 
In this manner, the fine second hard mask pattern 202b is obtained through the exposure of the resist films and the etching of the hard mask respectively performed twice. Therefore, the second hard mask pattern 202b thus formed by the double patterning can be used for dry etching, for example, the semiconductor substrate 201 (or a film to be etched (no shown) formed on the semiconductor substrate 201) for fine pattern lithography of the semiconductor substrate 201 (or the film to be etched).
In such a case, it is necessary for the hard mask 202 to have a given thickness sufficient for securing the dry etching resistance. Accordingly, when the second ArF resist film 206 is applied on the first hard mask pattern 202a as shown in FIG. 12B, it is apprehended that the application property may be degraded if the underlying first hard mask pattern 202a has a largely concavo-convex surface. In this case, the resolution attained in the second resist exposure is lowered, and hence, sufficient resolution cannot be attained although the double patterning is employed.
For overcoming such a problem, a method in which the first hard mask pattern 202a is planarized by using a BARC (bottom anti-reflection coating), that is, an anti-reflection film, is described in M. Maenhoudt et al., “Double Patterning scheme for sub-0.25 k1 single damascene structures at NA=0.75, λ=193 nm”, Proc. SPIE. vol. 5754, 1508 (2005).
Specifically, the first hard mask pattern 202a is planarized by applying a BARC 208 thereon as shown in FIG. 15, and the second ArF resist film 206 is applied on the BARC 208. Thus, the application property can be improved. As a result, the lowering of the resolution otherwise caused in the second resist exposure can be prevented.